Issue #4338, #3391 - Remove registered window class for the status bar

Issue #4338, #3391 - Remove registered window class for the status bar

[#4338] Remove application-specific window class for creating status bar
windows. Accordingly, remove 's_atomPanClass' and 'GetAtomPaneClass()', which
managed a class atom, and STATUS_BAR_PANE_NAME, the name of the ABP window
class. Instead, create windows using the system window class "Edit". Add window
names when creating them; this is useful for debugging and has no user-visible
effect.

Add 'SetWindowProc()', a type-safe wrapper for API 'SetWindowLongptr()' to
set window procedures. This replaces setting the window procedure by means of
a window class atom.

[#3391] Remove initialization code for the window class atom from
'InitObject()', the thread-main function for the detached initialization thread.

[Noissue] Move STATUS_BAR_PANE_NUMBER to PluginClass.cpp. This is an
implementation detail of the status bar and has no global significance.